1. Field of the Invention
The present invention relates to a surge suppression circuit for protecting an electrical load during surge events.
2. Description of the Related Art
A load control device is operable to control the amount of power delivered from an alternating-current (AC) power source to an electrical load, such as a lighting load or a motor load. Typical load control devices include, for example, dimmer switches for lighting loads, electronic ballasts for gas discharge lamps, light-emitting diodes (LED) drivers for LED light sources, and motor control devices for motor loads. Typical load control devices are designed to withstand surge events (i.e., transient spikes in voltage) that can occur on a power line as a result of inductive load switching, lightning strikes, and other transient events on the power line. Prior art load control devices have included voltage surge and voltage transient suppressors between the AC power source and the controlled electrical load to protect the electrical load from the surges and transient spikes.
In addition, typical load control devices are also designed such that the electrical insulation between the electrical circuitry and the outer enclosure (which is typically connected to earth ground) is appropriate, such that the load control devices do not provide unsafe conditions for a user. The insulation strength of a load control device may be tested using a “hipot” test in which the a large, low-frequency voltage (e.g., 2000 volts) is produced between the line voltage terminals and the outer enclosure (i.e., earth ground) and the leakage current through the insulation is measured to ensure that the leakage current does not exceed a safe magnitude.
FIG. 1 is a simplified schematic diagram of a prior art surge suppression system, which provides compliance with the ANSI standard for surge suppression. Hot and neutral power lines 10, 12 are coupled to electronic components 22 via a transformer 14, an RFI (radio-frequency interference) filter comprising capacitors 16A, 16B and magnetic core inductor 18, and bridge rectifier 20. A fuse 24 is coupled in series with the hot power line 10. A single clamping device, e.g., a metal-oxide varistor (MOV) 28, is coupled to the AC input terminals of the bridge rectifier 20 (i.e., after the RFI filter). The single MOV 28 provides protection from both differential mode (hot to earth ground) and common mode (hot and neutral to earth ground) surges. However, surge events can cause resonant ringing in the components of the RFI filter, which may greatly amplify the magnitude of the voltages applied to the electronic components 22. For example, the magnitude of the voltage across either of the capacitors 16A, 16B could ring up to approximately twice the magnitude of the surge voltage that is provided to the hot and neutral power lines 10, 12. Thus, a spark gap 26 is coupled from neutral to ground (in parallel with the capacitor 16B) to provide a controlled location for breakdown of the high voltage generated by the RFI filter. The surge suppression system of FIG. 1 is described in greater detail in U.S. Pat. No. 5,555,150, issued Sep. 10, 1996, entitled SURGE SUPPRESSION SYSTEM, the entire disclosure of which is hereby incorporated by reference.
Since the spark gap 26 is coupled from neutral to ground (in parallel with the capacitor 16B), the spark gap must be sized such that the system of FIG. 1 can pass the hipot testing, i.e., such that the spark gap does not breakdown at less than, for example, 2000 volts. Thus, the spark gap 26 negatively impacts the ability of the surge suppression system to limit the magnitude of the surge voltages applied to the electronic components 22. Therefore, there is a need for a surge suppression system that is able to pass hipot testing, while limiting the magnitude of the surge voltage applied to the electronic components 22 to even smaller levels.